The present invention relates to low speed, high torque gerotor motors, and more particularly, to such motors including a forward bearing package, or a forward brake package, or some other forward package which receives the dogbone (wobble) shaft which transmits the low speed, high torque output from the gerotor gear set.
A typical gerotor motor includes several housing sections disposed axially on opposite ends of a gerotor gear set. In most of the low speed, high torque gerotor motors which are currently in commercial production, the internally-toothed member (ring) is stationary, and the externally-toothed member (star) is disposed within the ring for orbital and rotational movement therein. Typically, such motors are provided with a dogbone (wobble) shaft, which transmits the rotational component of the movement of the star to the output of the device, such as an output shaft which comprises part of the forward package. It should be understood that, as used herein, the term "forward" is used arbitrarily to refer to the end of the device from which the output element extends, the output element typically being an output shaft or, in some cases, a rotatable housing or hub.
In typical gerotor motors of the type described above, the various housing sections are held in tight sealing engagement against the end faces of the gerotor gear set, and more specifically, with the end faces of the gerotor ring, by means of a plurality of bolts extending through one of the housing sections, then through the gerotor ring, and then into threaded engagement with the other housing section. See for example U.S. Pat. No. 3,270,681, assigned to the assignee of the present invention. In some motors, the heads of the bolts would be in engagement with the forward housing section and be in threaded engagement with the rearward housing section (also frequently referred to as the "endcap"), and in other motor designs, the heads of the bolts would be in engagement with the endcap, and would be in threaded engagement with the forward housing section.
In either case, conventional practice has been to provide seals (for example, "O" ring seals) between each adjacent pair of sections of the motor, with the seals being disposed radially inwardly from the bolts. Thus, in such an arrangement, the bolts are referred to as "dry bolts" because they are located radially outwardly of the "wet" region, i.e., the region of the motor within which, hopefully, the hydraulic fluid is retained.
It is also known to locate the bolts radially inwardly from the seals, in which case the bolts are referred to as "wet bolts". However, this has been considered generally less desirable because leakage fluid flowing between the adjacent surfaces of the various sections of the motor can flow into the bolt holes, which are typically somewhat oversized relative to the bolts, with such leakage then flowing axially along the bolts and then, possibly, to the exterior of the motor. As is generally well known to those skilled in the art, such leakage doesn't normally constitute a "flow" of fluid, but instead, typically constitutes a very small amount of fluid, frequently referred to as "weepage" because the fluid "weeps" through any gap or imperfection existing between the bolt head and the adjacent surface against which the bolt head is seated.
One of the primary performance criteria for a low speed, high torque gerotor motor is its volumetric efficiency, i.e., the actual rotational output of the motor as a percentage of the output of the motor which theoretically should occur, for a given flow of fluid through the motor. As is well known to those skilled in the art, the higher the internal (or external) leakage, the lower the volumetric efficiency, and vice-versa. It is also well known to those skilled in the art that volumetric efficiency is substantially reduced with increasing deflection of the various housing sections, axially away from the elements of the gerotor gear set, thus opening up larger leakage paths. It is recognized that one way of reducing deflection of housing sections away from the gerotor gear seat is to locate the bolts further inward radially, thus effectively making the housing sections "stiffer". However, moving the bolts further inward radially typically would result in "wet bolts" which, as was noted previously, is generally considered less desirable.